1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copier, and a facsimile machine, an image forming method, and a computer program product.
2. Description of the Related Art
An image forming apparatus, such as a copier and a laser beam printer, using an electrophotographic system performs the following image adjustment operation to obtain a stable image. That is, the electrophotographic image forming apparatus forms a test pattern on an image carrier such as a photosensitive element, and detects the test pattern on the image carrier with an optical sensor which is an optical detecting unit, and then adjusts image forming conditions, such as a charge potential, a developing bias, and an exposure amount, on the basis of a result of the detection.
There is a problem in the above-mentioned image adjustment operation, that is, the electrophotographic image forming apparatus can neither detect a defect in an image forming process (a process of transferring an image onto a transfer sheet and a process of fixing the image on the transfer sheet) performed on the downstream side of where the optical sensor is placed nor adjust the defect.
Japanese Patent Application Laid-open No. 2005-321572 discloses an image forming apparatus that includes a density sensor which is a toner-image detecting unit for detecting an unfixed toner image on an image carrier (including an intermediate transfer belt) and a color sensor which is a fixed-image detecting unit for detecting a fixed toner image fixed on a recording sheet, and detects a defect in an image forming process (a process of transferring a toner image onto a transfer sheet and a process of fixing the toner image on the transfer sheet) performed on the downstream side of where the density sensor is placed on the basis of results of detection by the density sensor and the color sensor. Specifically, first, the image forming apparatus forms an image adjustment pattern composed of a yellow gradation patch, a magenta gradation patch, a cyan gradation patch, and a black gradation patch under predetermined image forming conditions, and detects the image adjustment pattern with the density sensor. Then, the image forming apparatus forms another image adjustment pattern composed of the same gradation patches under the same image forming conditions, and detects this image adjustment pattern with the color sensor. Incidentally, the width of the image adjustment pattern detected by the color sensor is larger than that of the image adjustment pattern detected by the density sensor. It is considered that this is because of the following reason. The sensor for detecting a fixed image detects an image formed on a transfer sheet but does not differentiate between a diffusely-reflected light reflected from the surface of the transfer sheet and a diffusely-reflected light reflected from the image, so a general reflective optical sensor for detecting a toner adhering amount cannot be used as the sensor for detecting a fixed image. Consequently, as the sensor for detecting a fixed image, a color sensor including a white LED and a charge accumulation type sensor with an RGB on-chip filter is used. In such a color sensor, if a detection area is broadened, an amount of light input to the charge accumulation type sensor increases, and the detection sensitivity increases. Consequently, the detection area of the color sensor is broadened, therefore, it is considered that the width of the image adjustment pattern detected by the color sensor needs to be extended. On the other hand, as the density sensor for detecting a toner image on the image carrier, a general reflective optical sensor that receives a specularly-reflected light and a diffusely-reflected light which are reflected from the detection surface can be used because the surface of the image carrier is smooth. Then, a specularly-reflected light and a diffusely-reflected light which are reflected from the image adjustment pattern on the image carrier are detected, and a toner adhering amount of each of the patches is detected from a combination of the specularly-reflected light and the diffusely-reflected light. In such a reflective optical sensor, if a detection area is broadened, the sensitivity to a specularly-reflected light decreases; therefore, it is necessary to detect reflected lights with the detection area narrowed down. Consequently, there is no need to extend the width of each of the patches of the image adjustment pattern detected by the density sensor, therefore, it is considered that the width of the image adjustment pattern detected by the density sensor is smaller than that of the image adjustment pattern detected by the color sensor.
Then, the image forming apparatus checks whether a relation between the detection result of the density sensor and the detection result of the color sensor has a predetermined relation. If there is no defect in the transfer process and the fixing process, the relation between the detection result of the density sensor and the detection result of the color sensor is a predetermined relation; however, if there is any defect in the above processes, the relation is not a predetermined relation. Therefore, by checking a relation between detection result of the density sensor and a detection result of the color sensor, the image forming apparatus can detect a defect in the transfer process and the fixing process, and can adjust a transfer parameter and a fixing parameter on the basis of the detected defect in the transfer process and the fixing process. Furthermore, on the basis of the detected defect in the transfer process, the image forming apparatus adjusts an amount of toner adhered by adjusting a development parameter and a charging parameter, whereby it is possible to correct degradation of the image quality due to the defect in the transfer process.
However, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2005-321572, the main-scanning-direction position where the density sensor detects is different from the main-scanning-direction position where the color sensor detects. Therefore, the main-scanning-direction position where the image adjustment pattern detected by the density sensor is formed is different from the main-scanning-direction position where the image adjustment pattern detected by the color sensor is formed. As a result, the effect of main-scanning-direction uneven density of the detection result of the density sensor is different from the effect of main-scanning-direction uneven density of the detection result of the color sensor. Therefore, a relation between the detection result of the density sensor and the detection result of the color sensor is greatly affected by the effects of the main-scanning-direction uneven density, so the image forming apparatus cannot accurately detect a defect in the transfer process and fixing process.
To cope with this problem, in Japanese Patent Application No. 2009-39554, the present applicant has proposed an image forming apparatus in which a toner-image detecting unit and a fixed-image detecting unit are placed so that the main-scanning-direction positions of them are aligned. Consequently, the effect of main-scanning-direction uneven density of a detection result of the toner-image detecting unit and the effect of main-scanning-direction uneven density of a detection result of the fixed-image detecting unit can be about the same. Therefore, it is possible to prevent a relationship between the detection result of the toner-image detecting unit and the detection result of the fixed-image detecting unit from being affected by the effects of the main-scanning-direction uneven density. As a result, on the basis of the detection result of the toner-image detecting unit and the detection result of the fixed-image detecting unit, the image forming apparatus can accurately detect a state of an image forming process performed on the downstream side of the detection position of the toner-image detecting unit in a toner-image moving direction, and can perform highly-accurate image adjustment.
However, the image forming apparatus disclosed in Japanese Patent Application No. 2009-39554 performs only the image adjustment control using a detection result of the toner-image detecting unit and a detection result of the fixed-image detecting unit. Such image adjustment control using the detection result of the fixed-image detecting unit has the drawback of consuming a transfer sheet which is a user's belonging. If the execution interval of the image adjustment control is extended, it is possible to curb the consumption of transfer sheets due to the image adjustment control; however, a problem with the image quality may arise because the image adjustment control is not performed for a long time.